2D Ordering Phenomena Under Non-Equilibrium Conditions: An In Situ STM Approach

Diphenyl viologen on an HOPG electrode surface: less sharp redox wave than dibenzyl viologen

Redox behavior of diphenyl viologen (dPhV) on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode was described using the results of voltammetric and electroreflectance measurements. Its characteristics were compared to those of dibenzyl viologen (dBV), which undergoes the first-order faradaic phase transition. Unlike dBV, dPhV-dication (dPhV(2+)) was found to take a strongly adsorbed state on an HOPG surface. This is due to much stronger π-π interaction between phenyl rings of dPhV(2+) and HOPG surface than between benzyl groups of dBV(2+) and the surface. The participation of this strongly adsorbed dPhV(2+) in the redox process can be avoided by (1) a shorter than ∼3 min time period elapsing from touching a freshly cleaved HOPG surface to dPhV solution until the start of potential scan, (2) complete equilibration at the electrode potentials at which superficial dPhV molecules are fully reduced, or (3) multiple cyclic potential scanning to repeat oxidation-reduction of adsorbed species. Even in such conditions, although voltammograms of thin-layer electrochemistry for the surface-confined dPhV(•+)/dPhV(2+) couple are obtained with peak widths being as narrow as those of dBV, it is not the first-order phase transition. The participation of strongly adsorbed dPhV(2+) molecules results in another new voltammetric feature with a broader peak. The film formed by strongly adsorbed dPhV(2+) was hydrophilic, whereas dBV(2+) does not form such a film but only a gas-like layer. Measurements using X-ray photoelectron spectroscopy confirmed that the film consists of dPhV(2+) with coexistent water. These results reveal a typical case that delicate interaction balance among V(2+), V(•+), and electrode surface determines whether the two-dimensional first-order transition takes place or not.

2D analogues of the inverted hexagonal phase self-assembled from 4,6-dialkoxylated isophthalic acids at solid-liquid interfaces

Self-assembly of organic molecules at solid-liquid interfaces is a route for developing novel functional materials on surfaces and modeling assembly phenomena in 3D. 5-Alkoxylated isophthalic acids (ISA) are known to self-assemble into two-dimensional (2D) lamellae at the interface between a surface of Au(111) or HOPG (highly oriented pyrolytic graphite) and a solvent. Presently, the self-assembly of 4,6-dialkoxylated isophthalic acid derivatives with variable alkyl chain length is investigated at Au(111)-water, Au(111)-tetradecane and HOPG-tetradecane interfaces with a particular focus on the first one. The main aspect of this study is to evaluate the role of the molecular geometry and different interactions in the 2D assembly of amphiphilic molecules. In contrast to 5-alkoxylated ISA, 4,6-dialkoxylated ISA derivatives self-assemble preferentially into arrays of cyclic pentameric/hexameric structures, which appear as 2D analogues of the inverted hexagonal phase of lipids. As a general trend, the derivatives bearing shorter alkyl chains show a higher level of ordering at Au(111)-liquid interfaces. In particular, at the Au(111)-water interface, the 4,6-diheptyloxy ISA derivative forms exclusively pentamers, which are arranged in a quasi-hexagonal lattice. Moreover, the cyclic pentameric features are not empty but host a single isophthalic acid residue which is found to be dynamic. Finally, the packing of the diheptyloxy derivative shows a distinct potential dependence: while at more negative potentials the pentameric arrangement is converted into lamellae, at more positive potentials a loosely packed zig-zag pattern is formed. The present results show that at different solid-liquid interfaces 4,6-dialkoxylated ISA derivatives tend to form cyclic structures that are 2D analogues of an inverted hexagonal phase, akin to lipids having two hydrophobic alkyl chains and a small polar head group. Moreover, the substrate potential at the Au(111)-water interface can tune the 2D molecular arrangement.